FIG. 1 depicts a side-coupled standing-wave linear accelerator. This type of accelerator has been widely used in medical and industrial applications because it offers very high shunt impedance and operational stability. In order to increase shunt impedance per unit length, most of these accelerators use solely .pi./2 operational mode in the single section standing wave accelerator structure. For instance, the invention of the side coupled structure permitted elimination of a bend magnet and use of an extremely short in-line accelerator in a 360.degree. isocentric gantry for low energy radiation therapy machines. In this short standing wave linear accelerator structure, electrons 4 which are generated in the cathode 2 of the electron gun 1, are accelerated by DC voltage applied between the cathode 2 and the anode 7 and injected directly into the first cavity 3.
Since the applied voltage between the cathode 2 and anode 7 is only 10 to 30 kev, the velocities of these injected electrons are much slower than the velocity of light. As a result, the trajectories of the injected electrons depend strongly on the accelerating microwave electric field within the first cavity 3. The microwave power fed through the waveguide 25 generates an accelerating microwave electric field within the accelerating cavities 8. The microwave power is transmitted through apertures 5 of the coupling cavities 6 where accelerating cavities and coupling cavities are magnetically coupled through the aperture 5.
In order to efficiently couple these cavities magnetically, these coupling apertures are positioned away from the beam center where the electrons are accelerated. Due to the nature of these non-axisymmetric coupling apertures, the resultant accelerating electric field tends to offset from the beam centerline. These offsets may not be significant for the acceleration of the electrons, which have a velocity very close to the velocity of light, because the longitudinal momentum of high velocity electrons are much larger than the transverse momentum due to space charge affect and transverse accelerating fields. For the electrons injected initially into the first cavity 3, the trajectories will depend on the accelerating field within its cavity where coupling apertures are off-centered. Axisymmetric cavities excited with non-axisymmetric apertures tend to generate a non-axisymmetric electric field. As a result, the electrons accelerated in the first cavity tend to have non-axisymmetric electron distributions for a standing wave linear accelerator which uses only off-center magnetic coupling. This non-axisymmetric electron beam distribution generates non-symmetric Bremsstrahlung x-rays at the target 9 where normally very thin, but heavy metal (high atomic number)--such as tungsten--is imbedded into a water-cooled copper heat sink 10.
Another problem with this structure is that about two-thirds of the injected electrons are not accelerated in the first cavity because they are excited sinusoidally at the microwave frequency. Some of the electrons, which are not accepted in the first cavity, are often decelerated back to the electron gun, called back-bombardment, and damage the cathode of the electron gun.
Therefore, there is a need in the art for a linear accelerator having improved electron acceleration characteristics for compact side-coupled standing wave accelerators.